With development of mobile telecommunications systems and mobile telecommunications devices it is known to provide mobile phones with additional functionalities. These additional functionalities include digital cameras, mp3 players as well as satellite navigation based on GPS (Global Positioning System) system.
There are two main application of the satellite navigation implemented in mobile phones. In many situations it is desirable to locate users who are making emergency calls. This allows police, ambulance or fire brigade services to be dispatched to the location of the incident determined on the basis of the location of the user making a call. It is easier to understand the importance of this feature bearing in mind that in many cases the user making the call can be an accidental witness not knowing the exact location of the incident. In this scenario a network operator makes a request to the telephone via a network infrastructure and the telephone responds to the request. In the first step the GPS receiver of the phone is powered-up and then location of the phone is determined and finally transmitted this back to the operator. The other important application of the location functionality in a mobile phone is providing the user with details of his/her location. In this scenario it is the mobile phone user that makes the request to the positioning module of the telephone to determine its location. This can be useful, for example, when ordering a taxi or when the user wants to find out what, so called, points of interest (e.g. museums, clubs, etc.) are nearby.
Important factor that must be considered in designing portable devices like mobile phones equipped with GPS receivers is power consumption of the GPS receivers, which during GPS signal acquisition and tracking, and navigation processing can be high. In order to save the battery capacity the GPS receiver is switched off for most of the time and powered-up only when requested. The undesirable effect of that is tong period of time required before the position is determined, this is called time to first fix (TTFF).
Wireless signals concurrently transmitted from satellites at known locations are used in a process of calculating position of a GPS receiver. The GPS receiver receives the signals from satellites within its view of the sky. The GPS receiver measures the distance between itself and at least three GPS satellites by measuring time delay between transmission and reception of each GPS radio signal. Having the positions of three GPS satellites and distances between the GPS receiver and each of the three satellites the receiver calculates its position using trilateration. Accuracy of the calculations depends on accuracy of measurement of time. For the calculations it is necessary to know the time of transmission and the time of reception of the GPS signal. This allows calculating the distance using the known speed of the signal (speed of light). The clocks installed in satellites are extremely accurate and stable atomic clocks. The drawback of atomic clocks is that they are also very expensive. The clocks implemented in GPS receivers are based on quartz oscillators and they are not as accurate as the ones installed in satellites. Their accuracy can be affected by various factors like temperature, ageing, etc. Therefore, in order to obtain required accuracy the receiver's clock errors must be corrected (clocks must be synchronised)
One possible method, known in the art is to use a fourth satellite to synchronize the clocks. This method can be in many situations difficult to implement. First, it requires four satellites to be in line of sight of the receiver and the signals from all four satellites must be strong enough to allow for the calculation. In some environment, especially in urban areas where view of the sky is blocked by tall buildings it is not possible to receive signals from four satellites. When the signal is too weak it is impossible to demodulate it and retrieve from the signal all the necessary data (e.g. position of the satellite) without error. Second, the process of synchronization of clocks using four satellites is a very lengthy one and with customers demanding shorter and shorter times to first fix is not always suitable for determining location of a caller in emergency situations.
There is known in the art, European patent EP1229409 a solution that uses assistance data from a cellular telecommunications network in order to assist in synchronizing the clocks. However, operations as in the invention now to be described are neither disclosed nor suggested in the prior art document.
A disadvantage of the solution known in the art using assistance data is that it has an additional delay in scheduling a timing strobe from the radio access stack and in some cases the synchronisation is stopped as a result of cellular handover.
Hence, an improved device for determining location and a method of its operation would be advantageous and in particular one that allows reducing time to first fix.